In some types of communication networks, data stored in remote, data accumulation units are downloaded to a central computer or record keeping unit, periodically or on demand. For example, in electricity or other utility meter reading systems, to which the present invention is particularly advantageous but not necessarily limited, remote watthour meters are polled by a central computer which maintains customer electricity consumption records for billing. Typically there has been one centralized meter for each single-family home or for each apartment building occupied by a number of families, as well as for each single or multi-tenant commercial property, polled by a regional computer. It is preferable to monitor the electricity consumption of each user, rather than a community of users, to produce more equitable, individualized billing. However, it frequently is impractical to install separate meters in individual units, particularly in older apartment buildings, because there usually is no separate data communication line between each unit and a central site. Rewiring of the building is cost prohibitive. Accordingly, power lines provided to each unit and to which all meters are connected are used by the meters for transmitting electricity consumption data to a central computer. This is carried out by a transmitter in each meter which injects on the power lines a carrier signal modulated by data identifying the meter and reporting its consumption within a predetermined billing period.
However, power lines are electrically noisy so that it is difficult to establish a reliable communications link between the meters and the central computer using a reasonably small amount of signal power. Furthermore, the noise spectrum of the power lines changes with time of the day and season of the year depending on operation of electrical equipment connect, or located in proximity, to the power lines. For example, power line network characteristics have variations in attenuation as a function of frequency, with pronounced dips in transmission of some frequencies that vary among networks. Noise injected on the lines includes fixed frequency noise resulting, for example, from the switching of inductive loads. Other noise arises at harmonics of the frequency of the network, and there is gaussian background noise as well as fluctuating signals caused by televisions and the like operating on the power lines.
Accordingly, one object of the invention is to provide a method of and system for establishing reliable communications between local and remote data reporting equipment. Another object of the invention is to provide reliable power line communications method and apparatus suitable for centralized meter reading.
To improve reliability of data communications on the power lines, wide band transmission of digital information carried out using "frequency hopping" type spread spectrum signal processing wherein logical-one and logical-zero bits each encoded into multiple frequency elements or "hops" (sometimes called "chips") within two different frequency bands is disclosed in Galula et al. U.S. Pat. No. 4,763,103. Therein as described, at transmission each information element to be sent is encoded by a sequence of several different predetermined coding frequencies, wherein one sequence represents a logical-one bit and another represents a logical-zero bit. The two sequences of frequencies are within two different bands that are frequency spaced apart from each other. Within the two sequences, signals of equal rank, i.e. the same frequency position, must be different from each other to maintain a detectable degree of difference between the logical-one and logical-zero sequences. The sequences are demodulated at a receiver coupled to the power line by correlation with locally generated signals that produce, with a substantially constant frequency shift, the particular frequency combinations and the value of the information element being determined as a function of the correlation output.
Although generally satisfactory, the Galula et al. system is relatively complex, requires constraint in selection of frequencies forming the sequences and under certain circumstances may interpret received bits incorrectly. Complexity is a result of heterodyne signal demodulation that requires intermediate frequency conversion and amplification prior to correlation. The selection of frequencies is limited to frequencies within two different bands. Signal components which occupy one of the band are applied to one of the demodulators while the other components which occupy the other band are applied to the other demodulator, and as mentioned hereinabove the frequencies are not arbitrarily selectable since at least frequencies in corresponding ranks within the two bands must not be the same. Finally, a correlation decision is made in Galula et al. based on whether energy content following reception of each "chip" is above or below a predetermined threshold value. The value of a bit may become ambiguous and depend on the accuracy of the threshold if the number of decisions favoring one bit value or the other is near one half the total number of chips per bit. Accordingly, it becomes advantageous or necessary to limit the number of frequencies, or "chips", forming a bit to consist of an odd number of frequencies. In some cases, the decision may identify a logical-one or logical-zero bit incorrectly as noise.
A further object of the invention, therefore, is to provide a power line carrier communication method and system having improved immunity to line noise.
Another object of the invention is to provide more accurate bit detection in a frequency hopping type power line communication system.
A further object is to provide a frequency hopping type power line communication system that has simplified demodulation circuitry and wherein demodulation can be carried out in part by software.
Still another object is to provide a frequency hopping type power line communication system that has its frequencies distributed in a common band and minimum frequency selection constraints.